Individual Submission L. Dusseault
Internet-Draft OSAF
Expires: April 14, 2005 October 14, 2004
Partial Document Changes (PATCH Method) for HTTPdraft-dusseault-http-patch-06
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Copyright (C) The Internet Society (2004).
Abstract
Several applications extending HTTP require a feature to do partial
resource modification. Existing HTTP functionality only allows a
complete replacement of a document. This proposal adds a new HTTP
method, PATCH, to modify an existing HTTP resource.
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Internet-Draft HTTP PATCH October 20041. Introduction
Three use cases initially motivated this proposal
1. WebDAV [3] is used by authoring applications to store and share
files on the internet. For example, Adobe Photoshop has a
Workgroup feature allowing the user to browse a repository and
save the file. Currently, Photoshop only publishes the file to
the repository rarely, because Photoshop files are typically
large and upload is slow. Worse, large uploads are more likely
to be interrupted. Although HTTP [1] provides byte range
downloads, it does not provide a mechanism for partial uploads.
2. DeltaV [4] extends WebDAV to do versioning. In versioning
environments, a large number of files may be updated with very
small changes. For example, a programmer may change the name of
a function used in a hundred source files. Versioning
applications typically send deltas or patches to the server to
modify these files, however DetaV does not yet have this
functionality.
3. The SIMPLE WG is devising a way to store and modify configuration
information. The biggest feature missing from HTTP is the
ability to modify information in a very lightweight manner, so
that the client that decides to change its presence state from
"free" to "busy" doesn't have to upload a large document. This
can be accomplished through changes to a HTTP resource as well.
Other working groups (like netconf) are also considering manipulating
large files using HTTP GET and PUT. Sometimes the files aren't that
large but the device is small or bandwidth is limited, as when phones
need to add a new contact to an address book file. This feature
would allow much more efficient changes to files.
This specification defines a new HTTP 1.1 method to apply a delta
encoding, or a "patch", to a HTTP resource. A new method is
necessary to improve interoperability and prevent errors. The PUT
method is already defined to overwrite a resource with a complete new
body, and MUST NOT be reused to do partial changes. Otherwise,
proxies and caches and even clients and servers may get confused as
to the result of the operation.
Note that byte ranges are already used in HTTP to do partial
downloads (GET method) as defined in RFC2616. However, they are not
defined for uploads, and there are some missing pieces for uploads.
For example, the HTTP specification does not define a particularly
informative error to send if the byte range in a PUT is invalid.
Byte ranges (or some other kind of range) could be made to work in
this specification but a more flexible mechanism (one that could also
encompass XML delta encodings) was desired, as well as a method that
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Internet-Draft HTTP PATCH October 2004
would not confuse caching proxies. Reliable and tested delta
encodings already exist, and this specification takes advantage of
that existing work.
Some delta encodings for use in HTTP GET responses are defined in RFC3229 [2]. That specification defines delta encodings for cache
updates, not for user write operations. It does mean that servers
can reuse delta encoding algorithms to support both that
specification and this proposal.
This specification defines the new method PATCH to alter a single
existing resource, in place, by applying a delta encoding. A patch
request body is modelled as a manipulation of an instance, where the
instance would have been the entire document had it been PUT to the
server, following the model of RFC3229 [2]. The operation is atomic.
Note that WebDAV MOVE and COPY requests, if supported by the HTTP
server, can be useful to independently rename or copy a whole
resource before applying PATCH to either the source or destination
URL to modify the contents.
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Internet-Draft HTTP PATCH October 20042. Delta Encodings
A set of changes for a resource is itself a document, called a delta
encoding. The delta encoding is uniquely identified through a
instance manipulation as defined in RFC3229. Servers advertise
supported delta encodings for PATCH by advertising these algorithms,
and clients specify which one they're using by including the name in
the request. Not all instance-manipulations defined in the IANA
registry are delta encodings; as of October 2004, the instance
manipulations which are also delta encodings are vcdiff, diffe, and
gdiff.
Servers SHOULD support PATCH and the vcdiff delta encoding for all
authorable resources, that is all resources that support PUT. Some
requirements apply only to specific patch formats, and in this
specification those requirements are spelled out only for vcdiff.
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Internet-Draft HTTP PATCH October 20043. Mechanisms3.1 PATCH Method
The PATCH method requests that the request body (a delta encoding) be
applied to the resource identified by the Request-URI. The server
MUST NOT create a new resource with the contents of the request body,
although it MAY (depending on the delta encoding) apply the request
body to an empty entity to result in the content for the new
resource.
The server MUST always apply the entire patch atomically and never
provide (e.g. in response to a GET during this operation) a
partially-patched body. If the entire patch file cannot be
successfully applied then the server MUST fail the entire request,
applying none of the changes. See error handling section for details
on status codes and possible error conditions.
PATCH request bodies MUST NOT be cached. A cache MAY mark the
resource identified in the Request-URI as stale if it sees a
successful response to the PATCH request.
The PATCH request MUST have a body. It MUST include the IM header
with a single valid delta encoding. The PATCH request MAY include a
Content-Type header which is the content-type of the resource to
which the delta encoding is to be applied. The request body MUST be
in the delta encoding format specified in the IM header.
If the vcdiff format is used:
o The client MUST verify that it is applying the delta encoding to a
known entity. There are two reliable ways to do this. The first
way is to find out the resource ETag at the time the body is
downloaded, and use that Etag in the If-Match header on the PATCH
request to make sure the resource is still unchanged. The second
way to use WebDAV LOCK/UNLOCK to reserve the file (first LOCK,
then GET, then PATCH, then UNLOCK). Vcdiff collisions from
multiple users are more dangerous than PUT collisions, because a
vcdiff that is not operating from a known base point may corrupt
the resource. Therefore, if neither strong ETags nor LOCKS are
available from the server, the client MUST use If-Unmodified-Since
as a less-reliable safeguard.
o If the Request-URI does not identify an existing resource, the
server SHOULD (subject of course to access control and other
restrictions) create a resource with an empty body and apply the
vcdiff changes to that empty entity. A client SHOULD verify that
the URL is unmapped, as expected, with use of the "If-None-Match:
*" header.
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Internet-Draft HTTP PATCH October 2004
o The Content-Type header specifies the client's intended
Content-Type for the resource being patched. Thus, if the server
creates a new resource it MUST assign this Content-Type, or assign
a generic one if the Content-Type header was not provided. If the
server modifies an existing resource, the server MUST change the
Content-Type if a new Content-Type was provided by the client;
otherwise the resource's Content-Type should remain unchanged.
Simple PATCH example
PATCH /file.txt HTTP/1.1
Host: www.example.com
Content-type: text/plain
IM: vcdiff
If-Match: "e0023aa4e"
Content-Length: 100
[vcdiff-bytes]
Figure 1
This example illustrates use of the vcdiff algorithm on an existing
text file.
3.2 PATCH Response3.2.1 Success Response
A successful response with the 204 No Content status code implies
that no new resource was created. A successful response with the 201
Created status code informs the client that a new resource was
created.
The server SHOULD send the Content-MD5 header in responses to PATCH.
This allows the client to verify the success of the operation.
As with PUT, the PATCH method MUST change the resource's ETag if the
resulting entity is not identical to the original. If the server
supports strong ETags, the server MUST return a strong ETag for use
in future client operations. The server MUST return the
Last-Modified header if it does not support strong ETags.
Successful PATCH response to existing text file
HTTP/1.1 204 No Content
Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ==
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Internet-Draft HTTP PATCH October 2004
ETag: "e0023aa4e"
3.2.2 Error handling
This proposal uses the same mechanism as DeltaV (defined in section1.6 of RFC3253) to add machine-parsable information to provide more
detail than HTTP status codes can. Existing HTTP status codes are
not infinitely extensible but XML elements and namespaces are more
so, and it's simple to treat the HTTP error code as a rough category
and put detailed error codes in the body. Clients that do not use
the extra information ignore the bodies of error responses. These
error codes are not meant to be displayed directly to end-users, so
there is no language code or other display information. Clients MUST
ignore any unrecognized elements within the XML response body because
extensions allow implementors to add custom debug information to the
response.
The PATCH method can return the following errors. All these errors
are represented as XML elements in an XML document, where the
specific error element appears inside a root element called "error"
in the "DAV:" namespace. The new elements defined in this
specification are all in the "urn:ietf:params:xml:ns:patch"
namespace.
delta-encoding-badly-formatted: Used with 400 Bad Request when the
server finds that the delta encoding provided by the client was
badly formatted or non-compliant. The definition of badly
formatted or non-compliant depends on the delta encoding chosen,
but generally if the server finds it can't handle the current
patch even though it supports the format used, this error ought to
be appropriate.
delta-encoding-unsupported: Used with 501 Unsupported when the client
sends a delta encoding that the server doesn't support on this
resource, or a delta encoding that the server never supports.
patch-empty-resource: Used with 409 Conflict when the resource
addressed in the Request-URI exists but is empty, and the delta
encoding cannot be applied to an empty document. Note that some
delta encodings may be applied to an empty document, in which case
this error wouldn't be used.
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Internet-Draft HTTP PATCH October 2004
patch-result-invalid: Used with 409 Conflict when the resource could
be patched but the result of the patch would be a resource which
is invalid. This could mean, for example, that a XML resource
would become an invalid XML file if the patch specified that a
close element text line should be deleted.
"404 Not Found" can be used (with no body/error element) when the URL
in by the Request-URI does not map to a resource and the server
cannot apply the delta encoding to a new empty resource.
Other status codes defined in RFC2616 may also be used under the
appropriate circumstances, with no response body. For example, an
unauthenticated user may be prompted to authenticate, in order to use
PATCH, with "401 Unauthorized". An authenticated user who does not
have sufficient privilege to use PATCH may receive a "403 Forbidden"
response.
3.2.2.1 Example error response with body detail
HTTP/1.1 409 Conflict
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx
<?xml version="1.0" encoding="utf-8" ?>
<D:error xmlns:D="DAV:">
<P:patch-result-invalid
xmlns:P="urn:ietf:params:xml:ns:patch"/>
</D:error>
3.3 Advertising Support in OPTIONS
The server advertises its support for the features described here
with OPTIONS response headers. The "Allow" OPTIONS header is already
defined in HTTP 1.1 to contain all the allowed methods on the
addressed resource, so the server MUST add PATCH if it is allowed.
Clients also need to know whether the server supports special patch
formats, so this document introduces a new OPTIONS response header
"Accept-Patch". "Accept-Patch" MUST appear in the OPTIONS response
for any resource where the PATCH method is shown as an allowed
method.
OPTIONS * is not used to advertise support for PATCH because the
patch formats supported are likely to change from one resource to
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Internet-Draft HTTP PATCH October 2004
another. A server MAY include the Accept-Patch header in response to
OPTIONS *, and its value MAY be the union of known supported delta
encodings for all types of resources.
Accept-Patch = "Accept-Patch" ":" #instance-manipulation
Example: OPTIONS request and response for specific resource
[request]
OPTIONS /example/buddies.xml HTTP/1.1
Host: www.example.com
[response]
HTTP/1.1 200 OK
Allow: GET, PUT, POST, OPTIONS, HEAD, TRACE, DELETE, PATCH
Accept-Patch: vcdiff, gdiff, diffe, example-xcap-xml
The examples show a server that supports PATCH generally, with all
the delta encodings defined in RFC3229 plus one fictional
XML-oriented delta encoding. On some resources, for example on XML
files, different kinds of delta encodings more appropriate to the
resource may be supported.
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Internet-Draft HTTP PATCH October 20044. Interdependencies with other Standards4.1 PATCH and Access Control (RFC3744)
If the server supports WebDAV Access Control [5], then the PATCH
request SHOULD be subject to the same access control permissions as
the PUT request.
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Internet-Draft HTTP PATCH October 20046. Security Considerations
The security considerations for PATCH are nearly identical to the
security considerations for PUT. In addition, one might be concerned
that a document that is patched might be more likely to be corrupted,
but that concern is addressed through use of MD5 digests.
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Internet-Draft HTTP PATCH October 2004Appendix B. ChangesB.1 Changes from -00
OPTIONS support: removed "Patch" header definition and used Allow and
new "Accept-Patch" headers instead.
Supported delta encodings: removed vcdiff and diffe as these do not
have defined MIME types and did not seem to be strongly desired.
PATCH method definition: Clarified cache behavior.
B.2 Changes from -01
Removed references to XCAP - not yet a RFC.
Fixed use of MIME types (this "fix" now obsolete)
Explained how to use MOVE or COPY in conjunction with PATCH, to
create a new resource based on an existing resource in a different
location.
B.3 Changes from -02
Clarified that MOVE and COPY are really independent of PATCH.
Clarified when an ETag must change, and when Last-Modified must be
used.
Clarified what server should do if both Content-Type and IM headers
appear in PATCH request.
Filled in missing reference to DeltaV and ACL RFCs.
Stopped using 501 Unsupported for unsupported delta encodings.
Clarified what a static resource is.
Refixed use of MIME types for patch formats.
Limited the scope of some restrictions to apply only to usage of
required diff format.
B.4 Changes from -03
Various typographical, terminology consistency, and other minor
clarifications or fixes.
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Internet-Draft HTTP PATCH October 2004B.5 Changes from -04
Moved paragraphs on ACL and RFC3229 interoperability to new section.
Added security considerations.
Added IANA considerations, registration of new namespace, and
discontinued use of "DAV:" namespace for new elements.
Added example of error response.
B.6 Changes from -05
Due to various concerns it didn't seem likely the application/gdiff
registration could go through so switching to vcdiff as required diff
format, and to RFC3229's approach to specifying diff formats,
including use of the IM header.
Clarified what header server MUST use to return MD5 hash.
Reverted to using 501 Unsupported for unsupported delta encodings.
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Internet-Draft HTTP PATCH October 2004
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